#4 EFI Return lines

[nekit]

I am planning on using a EFI system on my O-320. I recently got a deal on a Andair fuel selector valve with #4 return ports. After the fact, I found out that EFII uses #6 on their returns to flow more fuel back to the tanks to limit vapor lock.

I decided to conduct a test to see how much difference can be had in this system. I know sizing is critical on the suction side, but not as much on the pressure side. I have a racing shop, so I had a lot of fittings and parts lying around. Here are the setups I tested:

#6 Return setup
6' of #6 braided line into Warbro pump, out #6 to pressure regulator set at 35psi, out to multiple #6 fittings(8), 5' of #6 hose and 5' #6 SS hard line.

Pumped gasoline for 30 sec into a container and measured the volume by weight in grams

Ran the test 4 times averaged 2695g or 86.4 fl/oz with a variable of about 10g

#4 test
Same setup, but after the regulator drop down to #4, with 8 fittings, 5' -#4 hose and 5'- #4 SS hardline.

Results averaged 2690g or 86oz

I difference of .4 fl/oz or 1/2 of a percent.

I'm planning on running all #6 lines to the regulator and only #4 hard lines after the regulator. I think I should be fine and plumbing may be a little easier with the #4 returns. I also believe the valve can be had for a fair amount less from Van's with the #4 fittings than you can find #6 ones elsewhere?

This isn't meant to start a debate and I may be off. I just thought I would share my findings.
Thanks

[Mark33]

I agree Rob, you should be fine. I would just double check with Robert and discuess your findings with him just to be on the safe side.

[BillL]

I was going to ask about the flow at 35ps, but if you maintained a constant 35 psi and that was the only volume drop it sounds insignificant. Even if it was an issue a simple adapter back up to #6 line could reduce losses as I suspect that most of the pressure drop in that return line is viscous losses in the total line and one fitting would dropped a size not be a major restriction delta. Line losses are generally linear with flow and fittings that act like orifices are (worst case) proportional to flow^2.

Good, real data, smart!

[rcpaisley]

You can probably do this, but there are some possible gotchas.
-4 teflon lines have an insert inside the teflon tube at the ends.
The hole in the insert is tiny - I have one in my hand, it is .154" ID.
So a -4 system may be much smaller in places than 1/4". If you avoid these pinch spots you will be OK.

Run -6 supply throughout. Use -6 return from the engine to the selector. Use a -6 to -4 reducer fitting into the selector on the return. Run -4 hard line back to the tank. This will keep a 1/4" minimum line size throughout the return. (don't use -4 teflon lines anywhere)

There is very little pressure on the return after the fuel pressure regulator. So the test performed on the -4 line may not represent data similar to the actual setup.

Robert